Android新增一个音频类型及双音频输出的实现
2016年01月18日 18:08:44 这歌声无聊可是辉煌 阅读数:7946
android定义了很多种音频类型,完整定义在native层如下,system\core\include\system\audio.h文件中:
/* Audio stream types */ typedef enum { /* These values must kept in sync with * frameworks/base/media/java/android/media/AudioSystem.java */ AUDIO_STREAM_DEFAULT = -1, AUDIO_STREAM_MIN = 0, AUDIO_STREAM_VOICE_CALL = 0, AUDIO_STREAM_SYSTEM = 1, AUDIO_STREAM_RING = 2, AUDIO_STREAM_MUSIC = 3, AUDIO_STREAM_ALARM = 4, AUDIO_STREAM_NOTIFICATION = 5, AUDIO_STREAM_BLUETOOTH_SCO = 6, AUDIO_STREAM_ENFORCED_AUDIBLE = 7, /* Sounds that cannot be muted by user * and must be routed to speaker */ AUDIO_STREAM_DTMF = 8, AUDIO_STREAM_TTS = 9, /* Transmitted Through Speaker. * Plays over speaker only, silent on other devices. */ AUDIO_STREAM_USB_HEADSET = 10, /* For accessibility talk back prompts */ AUDIO_STREAM_REROUTING = 11, /* For dynamic policy output mixes */ AUDIO_STREAM_PATCH = 12, /* For internal audio flinger tracks. Fixed volume */ AUDIO_STREAM_USB_MIC = 13, AUDIO_STREAM_ACCESSIBILITY = 14, AUDIO_STREAM_PUBLIC_CNT = AUDIO_STREAM_USB_MIC + 1, AUDIO_STREAM_CNT = AUDIO_STREAM_ACCESSIBILITY + 1, } audio_stream_type_t;
android为不同音频类型设置了不同的路由,根据路由选择不同的输出设备,这便是android的音频管理策略。
比如,应用层传入的音频类型是STREAM_MUSIC,插上耳机时,这种类型的声音会从speaker切换到耳机,如果音频类型是STREAM_RING,则会从耳机和speaker同时传出来。
AudioPolicyManager.h中定义了一下几种路由策略:
enum routing_strategy { STRATEGY_MEDIA, STRATEGY_PHONE, STRATEGY_SONIFICATION, STRATEGY_SONIFICATION_RESPECTFUL, STRATEGY_DTMF, STRATEGY_ENFORCED_AUDIBLE, STRATEGY_TRANSMITTED_THROUGH_SPEAKER, STRATEGY_ACCESSIBILITY, STRATEGY_REROUTING, STRATEGY_USB_HEADST, NUM_STRATEGIES };
根据路由为不同音频类型选择输出设备主要在AudioPolicyManager的getDeviceForStrategy方法,因此通过增加自定义音频类型和修改getDeviceForStrategy的音频策略,即可以对android的音频管理策略实现自定义。
例如实现这样的一个功能,在android智能电视上配合应用实现双音频输出的功能,即用户在看电视的过程中同时还可以听音乐,电视的声音从扬声器输出,而音乐的声音从耳机中输出,这里我们选择了一个usb 耳机设备。
实现原理即增加一个音频类型为音乐应用使用,打开双音频输出功能时,该应用传入的音频类型为我们自定义的,为该音频类型选择usb audio设备,同时,普通的tv及第三方应用使用的则是STREAM_MUSIC类型,该音频类型对应路由策略的是STRATEGY_MEDIA类型,我们在双音频功能打开的时候为该策略强制选择speaker设备,这样即实现了我们的双音频功能。
<pre name="code" class="csharp"> case STRATEGY_USB_HEADST: case STRATEGY_MEDIA: { char propDoubOutput[PROPERTY_VALUE_MAX]; property_get("audio.output.double_output",propDoubOutput,"null"); if ((strcmp(propDoubOutput,"1") == 0) && strategy == STRATEGY_USB_HEADST) { device = mAvailableOutputDevices.types() & AUDIO_DEVICE_OUT_USB_DEVICE; if (device != AUDIO_DEVICE_NONE) { device = AUDIO_DEVICE_OUT_USB_DEVICE; }else{ ALOGE("getDeviceForStrategy() no device found for STRATEGY_USB_HEADST"); } } else { uint32_t device2 = AUDIO_DEVICE_NONE; if (strategy != STRATEGY_SONIFICATION) { // no sonification on remote submix (e.g. WFD) if (mAvailableOutputDevices.getDevice(AUDIO_DEVICE_OUT_REMOTE_SUBMIX, String8("0")) != 0) { device2 = availableOutputDeviceTypes & AUDIO_DEVICE_OUT_REMOTE_SUBMIX; } }
STRATEGY_USB_HEADST类型是我们自定义的策略类型,"audio.output.double_output"为我们自己添加的一个属性,作为底层判断是否上层设置了双音频属性,可以看出在非双音频模式下,STRATEGY_USB_HEADST类型与MEDIA类型是完全一样的,在设置了双音频属性时,我们为 STRATEGY_USB_HEADST类型选择了usbaudio设备,device = AUDIO_DEVICE_OUT_USB_DEVICE;而同时我们还要为MEDIA设备选择speaker设备:property_get("audio.output.double_output",propDoubOutput,"null"); if (strcmp(propDoubOutput, "1") ==0) { device = AUDIO_DEVICE_OUT_AUX_DIGITAL |AUDIO_DEVICE_OUT_SPEAKER; } else { device |= device2; }
选择设备的工作基本就做完了,但是前提是需要j从ava层到framework层为该音频类型打通过程。实际上这个参照一种音频类型的实现就很容易解决。基本上理清一个audiotrack从java层到native层的调用过程即可,在java层audiomanger与audiosystem中添加我们自定义的音频类型之后来看audiotrack的构造函数,5.1之于4.4多了一个AudioAttributes,这对上层传下来的streamType做了一层封装,看上去是更方便了我们的扩展,通过上层stream_type转化得到 private int mUsage = USAGE_UNKNOWN;
和 private int mContentType = CONTENT_TYPE_UNKNOWN两种类型,到了native层AudioTrack.cpp的set函数中:
status_t AudioTrack::set( audio_stream_type_t streamType, uint32_t sampleRate, audio_format_t format, audio_channel_mask_t channelMask, size_t frameCount, audio_output_flags_t flags, callback_t cbf, void* user, uint32_t notificationFrames, const sp<IMemory>& sharedBuffer, bool threadCanCallJava, int sessionId, transfer_type transferType, const audio_offload_info_t *offloadInfo, int uid, pid_t pid, const audio_attributes_t* pAttributes) { ALOGI("set(): %p streamType %d, sampleRate %u, format %#x, channelMask %#x, frameCount %zu, " "flags #%x, notificationFrames %u, sessionId %d, transferType %d", this,streamType, sampleRate, format, channelMask, frameCount, flags, notificationFrames, sessionId, transferType); switch (transferType) { case TRANSFER_DEFAULT: if (sharedBuffer != 0) { transferType = TRANSFER_SHARED; } else if (cbf == NULL || threadCanCallJava) { transferType = TRANSFER_SYNC; } else { transferType = TRANSFER_CALLBACK; } break; case TRANSFER_CALLBACK: if (cbf == NULL || sharedBuffer != 0) { ALOGE("Transfer type TRANSFER_CALLBACK but cbf == NULL || sharedBuffer != 0"); return BAD_VALUE; } break; case TRANSFER_OBTAIN: case TRANSFER_SYNC: if (sharedBuffer != 0) { ALOGE("Transfer type TRANSFER_OBTAIN but sharedBuffer != 0"); return BAD_VALUE; } break; case TRANSFER_SHARED: if (sharedBuffer == 0) { ALOGE("Transfer type TRANSFER_SHARED but sharedBuffer == 0"); return BAD_VALUE; } break; default: ALOGE("Invalid transfer type %d", transferType); return BAD_VALUE; } mSharedBuffer = sharedBuffer; mTransfer = transferType; ALOGV_IF(sharedBuffer != 0, "sharedBuffer: %p, size: %d", sharedBuffer->pointer(), sharedBuffer->size()); ALOGV("set() streamType %d frameCount %zu flags %04x", streamType, frameCount, flags); AutoMutex lock(mLock); // invariant that mAudioTrack != 0 is true only after set() returns successfully if (mAudioTrack != 0) { ALOGE("Track already in use"); return INVALID_OPERATION; } // handle default values first. if (streamType == AUDIO_STREAM_DEFAULT) { streamType = AUDIO_STREAM_MUSIC; } if (pAttributes == NULL) { if (uint32_t(streamType) >= AUDIO_STREAM_PUBLIC_CNT) { ALOGE("Invalid stream type %d", streamType); return BAD_VALUE; } mStreamType = streamType; } else { <span style="color:#ff6666;"> // stream type shouldn't be looked at, this track has audio attributes memcpy(&mAttributes, pAttributes, sizeof(audio_attributes_t)); ALOGV("Building AudioTrack with attributes: usage=%d content=%d flags=0x%x tags=[%s]", mAttributes.usage, mAttributes.content_type, mAttributes.flags, mAttributes.tags); mStreamType = AUDIO_STREAM_DEFAULT;</span> } // these below should probably come from the audioFlinger too... if (format == AUDIO_FORMAT_DEFAULT) { format = AUDIO_FORMAT_PCM_16_BIT; } ......
看到 mStreamType = AUDIO_STREAM_DEFAULT; stream_type已经被设为-1,后面获取设备时不再关心stream_type,而是由audio_attributes_t这个结构体来选择,再来看看这个结构体的定义:
typedef struct { audio_content_type_t content_type; audio_usage_t usage; audio_source_t source; audio_flags_mask_t flags; char tags[AUDIO_ATTRIBUTES_TAGS_MAX_SIZE]; /* UTF8 */ } audio_attributes_t;
正是前面提到的mUsage 和mContentType 。
再回到AudioPolicyManager,看看getOutputForAttr接口,改接口调用了我们之前修改过的getDeviceForStrategy来获取设备:
......
ALOGV("getOutputForAttr() usage=%d, content=%d, tag=%s flags=%08x",
attributes.usage, attributes.content_type, attributes.tags, attributes.flags);
routing_strategy strategy = (routing_strategy) getStrategyForAttr(&attributes);
audio_devices_t device = getDeviceForStrategy(strategy, false /*fromCache*/);
......
所以在上层将stream_type 与AudioAttributes的转换做好,这条路就基本打通了,双音频输出的功能就实现了。